This invention refers to a process for improving the quality and especially the mechanical properties of single- or multi-ply sheets with a minimum of one ply consisting at least in part of thermoplastically processable starch, and to a single- or multi-ply sheet with a minimum of one ply containing at least in part thermoplastically processable starch.
Sheets produced from starch or polymer blends and with a content that includes starch are known. In addition, a series of polymer blends is known from the current state of the technology; their proposed uses include the production of sheets and they consist in part of starch.
The starting point for the increased use of starch in otherwise pure applications of plastics is, firstly, the desire to make increased use of renewable raw materials in contrast to the petrochemical or fossil basis of plastics, and secondly the requirement that plastic products produced from polymers should be biodegradable.
For example, filled polymers or polymer blends are known from EP-A-O 402 826, and they are produced by mixing natural starch or its derivatives with ethylene copolymers. In WO 90/14388 it is proposed that a polymer compound should be produced by mixing natural starch with polyethylene, adding either ethylene acrylic acid copolymers or glycerine. These polymer mixtures exhibit relatively poor mechanical properties even for the production of extruded mouldings and they are unsuitable for the production of sheets. It appears that the natural structure of starch is preserved in these blends at least in part, and this obviously detracts from their suitability as "Engineering Plastics".
In EP-A-O 327 505, EP-A-O 404 723, EP-A-O 407 350, EP-A-O 404 727 and EP-A-O 404 728, a series of polymer blends is proposed which is based on destructured starch and also contains a great variety of polymers or copolymers. Destructured starch is a modification of starch obtained from natural starch, having a moisture content of approximately 15-20%, i.e. the usual water content of natural starch, and is mechanically processed under heat. Research, particularly in connection with WO 90/05161, has shown that this destructured starch has only limited use as an "Engineering Plastic" because obviously natural starches are still present in this form of starch. This also appears to be the reason why the polymer blends proposed by the current technology--which are based on destructured starch--cannot be processed to make sheets that are free of defects. Sheets which are produced in this manner may have a brownish tint and also exhibit relatively poor mechanical values. In addition, the moisture resistance of such sheets is relatively poor.
EP-A-O 400 531 and EP-A-O 400 532 also propose that the starch should be destructured by means of a high-boiling plasticizing agent and a destructuring agent, which may take the form, for example, of glycerine used as a plasticizing agent and urea as a destructuring agent. Up to 15% ethylene acrylic acid and/or polyvinyl alcohol copolymers is added to starch which has been destructured in this manner. The plasticizing agent and the destructuring agent may enhance the destructuring effect in natural starch, but sheets produced in accordance with EP-A-O 400 531/32, both cited above, are not of the quality required for them to be used, for example, as packaging films with higher mechanical requirements.
For this reason an attempt has been made to start from the thermoplastically processable starch proposed in WO 90/05161 (corresponding to U.S. Ser. No. 07/785,931), which is almost entirely free of natural structural elements. The present document does not contain a detailed description of the production and characteristics of thermoplastically processable starch, but reference is made to WO 90/05161, the content of which is hereby incorporated in this description. Briefly, an essentially water-free starch is combined with an additive which lowers the melting point of the starch so that the melting point of the starch/additive mixture lies below the decomposition temperature of the starch, the additive having a solubility parameter of over 15 cal.sup.1/2 cm.sup.-3/2 and a vapor pressure lower than 1 bar at the melting range of the starch/additive mixture, and the resulting mixture is caused to melt by the application of heat and mechanical energy to realize an essentially moisture-free product.
Consequently, the as yet unpublished PCT/CH91/00078 (corresponding to U.S. Ser. No. 07/778,164) uses the abovementioned thermoplastically processable starch as the starch basis for the production of a polymer blend. In accordance with PCT/CH91/00078, the thermoplastically processable starch (TPS) is, for the purpose of producing the blend, mixed with a polyolefin, for example polyethylene or polypropylene. During the mixing it is preferable to add a phasing or bonding agent in the form of a block copolymer to facilitate the mechanical bonding between the starch and polyolefin phases. The sheets produced from this blend exhibit good mechanical properties and are highly moisture-resistant, although additional improvements still appear to be desirable and possible.